We are requesting money to purchase a JEM 100CX electron microscope to replace a 20 year old Phillips 200 in the electron miscroscopy facility of the Neurobiology Department. Electron microscopy has become an essential research tool for neurobiologists who must use multidisciplinary approaches to elucidate structure, function, development and disease in the nervous system. One reason for the importance of electron microscopy is that the nervous system consists of many diverse cell types whose morphological properties are strongly correlated with their functional properties. Meaningful interpretation of the results of biochemical and physiological experiments often depend upon knowledge of the intimate structural relationships of the cells involved. A second reason for the importance of the electron microscope is that the unequivocal morphological identification and mapping of the principal functional building block of the nervous system, the synapse, requires the resolution of the electron microscope. This instrument will be installed in an electron microscopy facility that has been in operation as a valuable resource of the Neurobiology Department for 20 years. It is essential to update this facility and provide resources for all the projects which depend upon ultrastructural work. The projects are broad in scope and include the identification of sensory transmitters, wiring in the visual system, mechanisms of modulation of neurotransmitter action and transmitter choice during development. These questions are central to our understanding of normal neuronal function and development and may help shed light on the disruption of function during disease.